Fault-tolerant magnetic bearing control system architecture

ABSTRACT

A fault-tolerant control system for a magnetic bearing arrangement which maximizes dynamic response of the control system is disclosed. The arrangement includes a main digital signal processor responsive to rotor position/velocity signals to supply control signals to bearing control coils and a secondary digital signal processor responsive to accelerometer signals to provide open-loop vibration control for correction of mass imbalance of the rotor system. In addition, sources of vibration which have fundamental frequencies at some multiple of the rotor system rotational frequency can also be controlled.

This is a divisional of copending application Ser. No. 08/862,816 filedMay 30, 1997 which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to a magnetic bearing control arrangementincluding a closed-loop rotor position control system and a separateopen loop vibration correction system. The use of multiple systems toperform separate tasks maximizes the dynamic response of the controlarrangement.

A number of papers have been published which discuss possible algorithmsfor implementing open-loop control methods for correcting rotor systemmass imbalance in rotating devices having magnetic bearings. Asdescribed in Knopse, C. R., Hope, R. W., Fedigan, S. J. and Williams, R.D., "Adaptive On-Line Rotor Balancing Using Digital Control,"Proceedings of MAG '93 Magnetic Bearings, Magnetic Drives and Dry GasSeals Conference & Exhibition, Alexandria, Va., Jul. 29-30, 1993,Technomic Publishing Company, Inc. and Ku, C. -P. R. and Chen, H. M.,"Optimum Shaft Balancing at a Rotor Bending Critical Speed with ActiveMagnetic Bearings", Proceedings of MAG '93 Magnetic Bearings, MagneticDrives and Dry Gas Seals Conference & Exhibition, Alexandria, Va., Jul.29-30, 1993, Technomic Publishing Company, these algorithms use signalsacquired from vibration transducers on the support structure for themagnetic bearings, as well as the rotor position sensors used by themagnetic bearing controller for closed-loop rotor position control, toderive sinusoidal perturbation signals that are applied to both thevertical and horizontal axes of each radial bearing in the system. Thesesinusoidal perturbation signals generally have equal amplitude andfrequency, but have a 90° phase relationship to each other and actsimilarly to a balance weight inserted into an imaginary balance planeat the same axial location as the radial bearing(s). Fedigan, S. J., andWilliams, R. D., "An Operating System for a Magnetic Bearing DigitalController," Proceedings of MAG '93 Magnetic Bearings, Magnetic Drivesand Dry Gas Seals Conference & Exhibition, Alexandria, Va., Jul. 29-30,1993, Copyright 1993 by Technomic Publishing Company, Inc. discussesreal time operating systems that might allow a single digital signalprocessing (DSP) unit to perform the continuous task of closed-looprotor position control for a multi-axis bearing system while stillleaving time between program loops to recalculate the necessaryamplitude and phase angle for the open-loop sinusoidal perturbationsignals.

However, a system using two separate DSP units; one for the highpriority task of closed-loop rotor position control, the second for thelower priority but computationally intensive task of recalculatingamplitude and phase for sinusoidal perturbation signals, would avoid anypriority conflicts and has performance advantages over a single DSPunit. This type of system, coupled with a homopolar bearing as disclosedin U.S. Pat. No. 5,111,102 to Meeks, or other bearings having redundantcontrol coils, could also be used to implement a completely redundantmagnetic bearing system/controller with both digital closed-loop rotorposition control and open-loop mass imbalance correction and harmonicvibration control.

SUMMARY OF THE INVENTION

Accordingly it is an object of the invention to provide a magneticbearing control system that incorporates closed-loop rotor positioncontrol as well as open-loop rotor system mass imbalance correction andharmonic vibration control using multiple control signal processingunits to perform separate tasks so as to maximize dynamic response ofthe control system.

A further object of the invention is to integrate the aforementionedcontrol system arrangement into a fault tolerant magnetic bearing systemconfiguration where complete redundancy is provided for all componentsof the system up to and including the control coils and rotor positionsensors in the magnetic bearings.

These and other objects of the invention are attained by providing abearing arrangement for rotating devices utilizing a magnetic field forlevitating and controlling the radial and axial positions of a rotorcomprising a control unit including a closed loop rotor positioncontroller and separate open loop vibration correction device. Theclosed loop rotor position unit includes one enabled closed-loop rotorposition controller and one disabled closed-loop rotor positioncontroller. The open loop vibration correction device simultaneouslyenables the disabled closed-loop rotor position controller and disablesthe enabled closed-loop rotor position controller.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will be morefully appreciated from a reading of the following detailed descriptionwhen considered with the accompanying drawings wherein:

FIG. 1 is a schematic cross-sectional view illustrating a conventionalradial magnetic bearing arrangement;

FIG. 2 is a perspective view illustrating a conventional 5-axis magneticbearing/rotor arrangement;

FIG. 3 is a schematic block diagram showing a representative embodimentof a digital signal processor based magnetic bearing control arrangementusing both closed-loop and open-loop control in accordance with theinvention; and

FIG. 4 is a schematic block diagram showing a representative embodimentof a fault tolerant magnetic bearing arrangement in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A conventional radial magnetic bearing configuration is shown in FIG. 1.The control coils 1, 2, 3 and 4 are used to generate magnetic flux 5, 6,7 and 8 that couples a ferromagnetic machine rotor journal 9 and aferromagnetic stator 10. The control coils are arranged such that eachis capable of exerting a force on the rotor journal in a direction whichis radially outward in their respective quadrants. The bearing alsoincorporates rotor position sensors 11, 12 which are typically of theinductive or capacitive type, and provide a feedback signal to acontroller that generates control currents 13, 14, 15 and 16 that passthrough the control coils 1, 2, 3, and 4. The magnetic flux generated bythe control coils acts to bring the rotor journal to some targetposition. The control coils may be constructed such that two or moreseparate conductors are wound so as to link a common flux pathinfluencing a given axis of the bearing. Each conductor has anindependent current source and is controlled by one or more closed-looprotor position control circuits. This allows control of a given bearingaxis to be maintained even if an individual conductor in the controlcoil for that axis or its associated current source were to fail. Thismethod can also be applied, in general, to other active magnetic bearingconfigurations, since most use control coils to vary the intensity ofmagnetic flux influencing the machine rotor on a given axis.

A conventional 5-axis rotor system is shown in FIG. 2. The rotor systemconsists of the machine rotor 17, which is supported by two radialbearings 18 and 19 and constrained axially by a thrust bearing 20. Theradial bearings 18, 19 each have at least two axes of control which aretypically oriented 90° apart in the vertical and horizontal planessimilar to the configuration shown in FIG. 1. The thrust bearing 20consists of two opposed electromagnets 21, 22 acting on a thrust collarrigidly attached to the machine rotor. The machine rotor will typicallyhave some degree of mass imbalance at one or more axial locations 23, 24and may experience reaction forces that may be steady state (e.g.,gravitational) or dynamic (e.g., fluid flow pulsations, electrodynamicpulsations, etc.)

A magnetic bearing control system architecture in accordance with theinvention which incorporates both closed-loop rotor position control andopen-loop control signal generation for correction of mass imbalance andother dynamic rotor loads is shown schematically in FIG. 3. While FIG. 3illustrates an arrangement in which a DSP unit 25 is used to generatethe closed-loop rotor position control signals, there are many analogcontroller architectures, for example, as described in Kipp, R. andImlach, J., "Control Methodology for Commercial Magnetic BearingSystems" Proceedings of the PCIM/Power Quality Conference, Irvine,Calif., Sep. 20-24, 1992, Copyright 1992 by Interfee International, Inc.that could also be used. The simplified program loop shown for the mainDSP unit 25 is for illustration purposes only and as understood by thoseof skill in the art would likely be replaced with a much moresophisticated feed-forward linear estimator type algorithm in an actualsystem implementation. In the closed-loop rotor position controlalgorithm shown in FIG. 3 the main digital signal processor 25 isprovided with a numerical array of actual rotor positions [RPA] andvelocities [V] which are obtained from rotor position and velocitytransducers located at each magnetic bearing axis. These transducersprovide analog rotor position/velocity signals 26 which areappropriately conditioned and converted to numerical values by ananalog-to-digital converter 27 and stored in the numerical arrays [RPA]and [V] which reside in a volatile RAM 28.

The main digital signal processor also uses a number of externallydefined parameters to calculate the closed-loop rotor position controlsignals. For the illustrative system shown in FIG. 3, these parametersare entered via an external computer interface 29, e.g., a portablecomputer connected to the controller via a serial communicationinterface, and reside in a non-volatile RAM or other suitable memorydevice 30. The example parameters shown in FIG. 3 include an array [RPT]containing target rotor positions for each bearing axis, an array [K]containing linear stiffness coefficients for each bearing axis, an array[C] containing linear damping coefficients for each bearing axis, alogical array [pvcmode] identifying whether programmed vibration controlmode is "on" or "off" for each given bearing axis, a set of twodimensional numerical arrays containing programmable notch filterparameters to be applied to control signals for bearing axes that have[pvcmode] ="on" that include center frequency of the notch filter [f_(m)], filter gain [G_(m) ], and filter quality or "Q-factor" [Q_(m) ].Other parameters residing in the non-volatile RAM 30 are not used by theclosed-loop rotor position control section and will be discussed laterherein.

The simple control algorithm shown in the program loop block for themain digital signal processor 25 starts by calculating the rotorposition errors [e] by subtracting the actual rotor positions [RPA] fromthe target rotor positions [RPT] for all bearing axes. The rotorposition errors [e] are then multiplied by the linear stiffnesscoefficients [k], and added to the rotor velocities [v], multiplied bythe linear damping coefficients [C] to obtain the unfiltered controlsignals [i]. The rotor velocity [v] may be estimated from changes in [e]over previous time intervals, since [v] represents the rate of change of[e] with respect to time. Although this is a commonly used technique,obtaining rotor velocity using a dedicated velocity transducer reducescomputational demands on the digital signal processor. Whether [v] isobtained by differentiating [e] or directly using a velocity transducer,the control algorithm shown in the program loop block for the maindigital signal processor [25] uses a conventionalproportion-integral-derivative (PID) approach, and as such, can beimplemented by performing all the mathematical operations describedusing analog operational amplifiers. However, digital processing doespermit more sophisticated mathematical operations such a non-linear andhigh-order polynomial type functions to be implemented without theextensive proliferation of electronic hardware and signal noiseassociated with analog circuitry.

As discussed earlier, it is also common practice to use a state spacetype algorithm to improve the accuracy of the control signal calculationbeing performed by the main digital signal processor 25. This is usuallydone by using one or more previous values of [e] and/or [v] stored inmemory to perform a linear regression that can be used to extrapolatewhat the values of [e] and [v] will be at the time the next set ofcontrol signals [i] will be written to an output buffer 31. The timeinterval at which the [i] values in the output buffer 31 are updated istypically fixed so that the exact time at which the extrapolatedestimates of [e] and/or [v] are needed, is known with a great deal ofprecision. This estimation process minimizes error introduced as theactual rotor positions and velocities at each bearing axis change overthe time period from when [RPA] and [v] values are obtained to when the[i] values are calculated and written to the output buffer 31. Thelinear estimator approach to minimizing this error can be furtherimproved by incorporating a simple mathematical model of the machinerotor dynamic response in place of the simple linear extrapolation. Thislatter method is computationally more insensitive than a linearestimator but is also commonly used and typically results in somecompromise between increased computational burden on the digital signalprocessor and reduced error in the state space estimator.

For most digital signal processor applications, it is possible to extendthe time interval at which the output buffer 31 is updated to a periodlong enough to perform all PID-type closed-loop rotor position controlsignal [i] related calculations with time left over to implement notchfiltering to attenuate the [i] signal over narrow frequency ranges whereexcitation of resonances in the machine rotor or support structure mustbe avoided. For typical systems, the control signal generated for eachbearing axis may require several notch filters set at variousfrequencies with various gains, or may not require any notch filteringat all, depending on the resonant response characteristics of themachine rotor and support structure. Therefore, the main digital signalprocessor 25 program loop incorporates a conditional statement that onlyexecutes the digital filter algorithm H ([f_(m) ], [G_(m) ], [Q_(m) ],[i]) if the programmed vibration control mode status indicator [pvcmode]is set to "on" for a given bearing axis, thereby limiting computationaldemands on the main digital signal processor.

In addition to the closed-loop rotor position control function beingperformed by the main digital signal processor 25, a secondary digitalsignal processor 32 is incorporated in the control system to performopen-loop vibration control functions. These open-loop functions includean automatic balance mode (abmode) and an adaptive vibration controlmode [avcmode] which can be activated via a user interface 33, whichtoggles the mode status indicators "on" or "off" in a volatile RAM area34.

The secondary digital signal processor 32 uses signals generated byaccelerometers 35 or other vibration measurement devices, that areappropriately conditioned and converted to a numerical array of values[a_(nn) ] by analog-to-digital converters 36 and stored in a volatileRAM 37. The open-loop control signals are generated in the form of anumerical array of amplitudes [A_(n) ] and phase angles [Θ_(n) ] whichdefine sinusoidal perturbation signals that are superimposed on theclosed-loop control signal [i] being generated by the main digitalsignal processor. These sinusoidal signals are generated independentlyfor each bearing axis. The signal have frequencies equal to therotational frequency of the machine rotor up to and including the nthharmonic of this frequency or have frequencies which are fractionalmultiples of the rotational frequency. The phase angle array [Θ_(n) ]provides an angular reference for the sinusoidal signals at higher thanrotational frequency relative to a known reference on the machine rotorwith 360° corresponding to one full cycle at the harmonic frequency,i.e., one complete revolution of the machine rotor.

The [A_(n) ] and [Θ_(n) ] values are stored in a non-volatile RAM 38.The [A_(n) ] values are then compared by a digital comparator 39 withmaximum amplitude values [(A_(n))max)] which are defined via theexternal computer interface 29. The [A_(n) ] values reside in thenon-volatile RAM 30, and may be limited if necessary. This precludesperturbation signals of excessive amplitude from being superimposed onthe closed-loop rotor position control signals. The [A_(n) ] and [Θ_(n)] values are then sent to a high-frequency digital waveform synthesizer40 which uses a digital tach/phase reference 41 to produce a separatecomplex digitized waveform to be superimposed on the closed-loop controlsignal [i] for each bearing axis. The complex digitized waveforms arethe sum of all the sinusoidal perturbation signals being applied to eachbearing axis and are combined with closed-loop control signals [i] by ahigh-speed digital adder 42. The output from the adder is sent to adigital-to-analog converter 43, followed by anti-aliasing filters 44 orother signal conditioning. The conditioned analog control signals arethen sent to power amplifiers 45 followed by bearing control coils 46.This results in some type of rotor/bearing response 47 that is in turnsensed via the rotor position/velocity signals 26 and the accelerometers35.

The secondary digital signal processor can generate the amplitude [A_(n)] and phase [Θ_(n) ] values in a manner similar to that commonly used inconventional mechanical rotor balancing techniques. During mechanicalbalancing, a trial weight of known mass is placed at a known radial andangular location in a given balance plane on the machine rotor. Anysuitable trial weight mass and location may be selected. This additionof a trial weight causes an angular deflection and change in amplitudeof the rotational signal of the machine rotor at a given operatingspeed. These changes are detected by an accelerometer or other vibrationdetection means on one or more of the machine bearing housings orsupport structure. If the original imbalance of the machine rotor andthe associated acceleration as measured on the bearing housing orsupport structure are represented by phasors m₀ and a₀, and theimbalance with the trial mass added and its associated bearing structureacceleration represented as phasors (m₀ +m_(t)) and a_(t), then a systemof two phasor equations can be written:

    a.sub.0 =c.m.sub.0

    a.sub.t =c.(m.sub.0 +m.sub.t)

This system has two unknowns; m₀, which is the original rotor imbalanceand c, which is sometimes referred to as an influence coefficient. Oncethe original rotor imbalance phasor m₀ is determined, an exactcorrection mass and location can be calculated to correct for theimbalance. Once the influence coefficients of each balance plane foreach location of interest on the machine structure have been determined,vibration levels at any of these locations can be adjusted. Thiscomplete set of influence coefficients, each of which are phasorquantities, is generally represented in an n×k matrix where n representsthe number of balance planes on the machine rotor where balance weightscan be added and k represents the number of locations on the machinestructure where the influence of the balance planes is determined. Thevalues in the influence coefficient matrix are also sensitive torotational speed since the modal response of the machine rotor andmachine structure will be dependent on the frequency of the forcingfunction, which in mass imbalance calculations is the rotationalfrequency of the rotor.

This type of influence coefficient matrix, which can be considered to bea type of transfer function, can be generated for a radial magneticbearing of the type shown in FIG. 1. By injecting a sinusoidal signal ofknown amplitude (A₁)₁, and a phase angle (Θ_(n)), relative to a fixedreference on the machine rotor on one control axis, e.g., the verticalaxis of the bearing, and another sinusoidal signal of the same amplitude(A₁)₂ to the other control axis, e.g., the horizontal axis of thebearing, with a phase angle shifted 90° in the direction of rotorrotation from the sinusoidal signal sent to the first control axis, theeffect of adding a balance weight at an imaginary balance plane at theplane of the bearing can be simulated. However, the magnitude ofexcitation is simply A₁, rather than the product of a trial mass and itseccentricity. Therefore, for each radial magnetic bearing, an influencecoefficient can be generated for any location of interest on the machinestructure. This method of generating an influence coefficient matrix mayalso be extended to axial bearings or even a single axis of a radialbearing. In that case only one sinusoidal perturbation signal isrequired, instead of two at 90° apart as described above forsimultaneous excitation of a vertical and horizontal control axis.

Additionally, this method can be used to determine influencecoefficients at harmonics of rotational frequency. This is useful inmany types of machines such as electric motors or generators wherevibration may be caused in the motor/generator frame by electricalexcitation at multiples of the fundamental electrical frequency, E, suchas the frequencies 2×E and 6×E. For two pole motors/generators, thesecorrespond to twice and six times the rotational frequency of the motor.For motors or generators with more than two poles, these electricalexcitations will occur at higher harmonics of the rotational frequency.Therefore, by matching the frequency of sinusoidal perturbation signalsto the frequencies of these excitations which occur at other thanrotational frequency of the rotor, unwanted vibration in the structureof the machine can be reduced, provided at least one bearing hassufficient influence over the location on the structure of interest.

In the arrangement shown in FIG. 3, the open-loop perturbation signalsare added to the closed-loop rotor position control signal using a highspeed digital adder 42. The output buffer 31 is only changed by theclosed-loop rotor position control loop a few times per revolution ofthe machine rotor while hundreds or thousands of values from thehigh-frequency digital waveform synthesizer 40 may be generated duringeach revolution of the rotor. This allows perturbation signals to besuperimposed on the closed-loop rotor control signal at harmonics thatare many times the rotational frequency. Conventional magnetic bearingcontrol systems that incorporate open-loop control features calculateand sum the perturbation signal within the closed-loop controlalgorithm. This limits the frequency at which open-loop perturbationsignals can be injected to rotor rotational frequency, making massimbalance and other low frequency excitation the only sources ofvibration that can be acted on by open-loop signals.

A magnetic bearing control system architecture that allows theclosed-loop rotor position control and open-loop perturbation signalgeneration features shown in FIG. 3 to be implemented in afault-tolerant configuration is shown in FIG. 4. A machine rotor 48 isshown passing through a single radial bearing of the type disclosed byMeeks in U.S. Pat. No. 5,111,102. The more conventional radial baringconfiguration shown in FIG. 1 may also be used. The hybrid bearingconfiguration shown in FIG. 4 is particularly amenable to fault-tolerantcontrol system configurations, however, because left side and right sidecontrol coils 49 and 50 can each be sized to individually control thegiven bearing axis with relatively small change in the bearingdimensions for a given load rating. The bearing is equipped with dualrotor position sensors 52 on each axis, with each sensor havingdedicated signal conditioning 53 and analog-to-digital converters 54 and55. The digitized rotor position signals are then sent to primary andstandby DSP based on controllers 56 and 57. The primary and standbycontrollers 56 and 57 are enabled or disabled by a DSP based supervisorcircuit 58 which receives digitized input signals from ananalog-to-digital converter 59. This analog-to-digital converter 59digitizes vibration signals measured by an accelerometer 60 sent via asignal conditioning unit 61.

The DSP supervisor is programmed to detect excessive vibration on thebearing or support structure. If a vibration level exceeds a specifiedsafe or desired limit, the active primary DSP based controller 56 isdisabled and the inactive standby DSP based controller 57 is enabled.Using vibration level as an independent parameter to detect amalfunctioning controller and implementing logic which prevents morethan one controller from being active at a time, e.g., an inverter 64,ensures that a malfunctioning controller will not generate controlsignals which interfere with the back-up controller. Other ways ofisolating a malfunctioning or inactive standby controller, such asoutput isolation, may also be used.

The DSP based supervisor 58 is also programmed to use the digitizedvibration signals from the accelerometer 60 to generate open-loopperturbation signals which are stored in memory areas 62 and 63. Thesememory areas are analogous to the non-volatile RAM 38 shown in FIG. 3.The DSP based controllers 56 and 57 each incorporate all of the featuresshown on FIG. 3 required for closed-loop rotor position control. Inaddition each of the DSP based controllers has a digital comparator 39,high-frequency digital waveform synthesizer 40, digital tachometer/phasereference 41, and the high speed digital adder 42. This allows eitherDSP based controller to maintain open-loop control functions should theother malfunction. A serial communication interface 65 allows thevarious parameters in the non-volatile RAM 30 shown in FIG. 3 to beloaded into each DSP based controller 56 and 57.

Two pulse-width modulated (PWM) amplifiers 66, 67 can receive controlsignals from either DSP based controller 56 or 57. The PWM PowerAmplifier 66 drives the right side control coils 50 while the left sidecoils 49 are driven by the PWM Power Amplifier 67. Each amplifier drivesthrough fuses or other passive overcurrent protection devices 68.

Redundancy in the power supply section is provided by incorporating twoindependent DC power supplies 69 and 70 driving through diodes so thatif one unit fails, DC power to the circuit is maintained. A similarconfiguration is used for two logic level, e.g., 5 VDC power supplies 71and 72. The main power supplies 69 and 70 are each connected toindependent buses, BUS 1 and BUS 2, to provide further redundancy. TheDSP based controllers 56 and 57, DSP based supervisor 58, PWM poweramplifiers 66 and 67, DC power supplies 69 and 70, and logic level powersupplies 71 and 72 each incorporate fault indication outputs F which arefed into a fault detection and indication logic/display 73, which issimply a set of lights or other type of enunciator that indicates when aparticular portion of the system has suffered a fault.

The arrangement shown in FIG. 4 would typically be used to operate alarge number of bearings, each having the features shown in the boundarylabeled "common to each axis of the 5 axis controller (74)". Asrecognized by those skilled in the art the arrangement can be used tooperate different bearing configurations as well. A rotor system, suchas that shown in FIG. 2, typically has 5 axes, consisting of two radialbearings, each having a vertical and horizontal control axis, and athrust bearing which controls the axial position of the rotor. Rotorsystems and magnetic bearing system controllers may also be designedwhich have fewer or a greater number of axes. Each 5-axis controllercontains the features shown in FIG. 4 within the boundaries labeled"common to each 5-axis controller (one or more per enclosure) (75)." The5-axis controller may be installed in an enclosure along with four powersupplies 69, 70, 71 and 72 and a fault indication unit 73 shown in FIG.4 within the boundaries labeled "common to each controller enclosure(76)."

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations will readilyoccur to those skilled in the art. Accordingly, all such variations andmodifications are included within the intended scope of the invention.

I claim:
 1. A method for correcting imbalance in a rotor device havingmagnetic bearings comprising:providing a bearing arrangement comprisingcontrol means including closed loop rotor position means comprising afirst signal processing means and separate open loop vibrationcorrection means comprising a second signal processing means whichgenerates an open loop perturbation signal and vibration sensing meansfor detecting when vibration of a stationary portion of the bearingarrangement exceeds a predetermined level wherein the closed loop rotorposition means includes one enabled closed-loop rotor positioncontroller means and one disabled closed-loop rotor position controllermeans; providing at least two independent position sensor meansdedicated to each of the enabled closed-loop rotor position controllermeans and one disabled closed-loop rotor position controller means foreach axis along which the axial position of the rotating members iscontrolled; and simultaneously enabling the disabled closed-loop rotorposition controller means and disabling the enabled closed-loop rotorposition controller means with the open loop vibration correction meanswhen vibration of a stationary portion of the bearing arrangementexceeds a predetermined level.
 2. A method for correcting imbalance in arotor device having magnetic bearings in accordance with claim 1 furthercomprising providing input signals from each enabled closed-loop rotorposition controller means to at least two independent poweramplification means associated with each axis of the bearing arrangementand providing control current from each power amplification means to adedicated control coil therefor, wherein the control coil links a commonflux path associated with a given bearing axis along which rotorposition is controlled.